In the manufacture of microelectronic devices, particularly VLSI and ULSI devices, in which the overall dimensions of devices are becoming bigger and more closely packed, and the critical dimensions are becoming much smaller, particle contamination is of increasing concern. The presence of particles during deposition or etching of thin films can cause voids, dislocations or shorts which adversely affect performance and reliability of the devices.
The problem of contaminant particles was addressed initially by improving the quality of clean rooms and improving automated equipment to handle materials and semiconductor substrates, e.g., silicon wafers. Improved cleaning of wafer surfaces was also addressed. These improvements have greatly reduced the number of particles that are present or fall onto a wafer surface during the processing cycle. However, many particles are generated inside the wafer processing chambers themselves due to the materials used for processing and due to mechanical means, for example the rubbing movements of robotic equipment during transfer operations and the like.
In particular during processing that uses a plasma, many fragments of various kinds are generated from the processing gases, including ions, electrons, particles, and the like. The fragments can combine to form generally slightly negatively charged particles, i.e., on the order of 10.sup.4 negative charge. In addition, the processing chamber becomes coated with various materials, such as polymers, during plasma processing of a wafer. Stress, such as stress due to thermal cycling and the like, causes these films to fracture and dislodge from the walls and surfaces on which they have deposited, also generating particles. In addition, particles are generated within the processing chambers during wafer transfer operations, pump oil backstreaming and the like.
Selwyn et al have disclosed one solution to avoiding deposition of particles onto a wafer. They redesigned the wafer support electrode to provide grooves therein which provide a disturbance around the periphery of the wafer, and beyond the wafer surface. This attracts particles to this disturbed region beyond the wafer so that the vacuum chamber exhaust system can carry particles away from the wafer and outside of the processing system. While a reduction of particles of up to 70% is said to be obtained, it does require redesign of wafer support electrodes and retrofitting of existing equipment, which is expensive.
Selwyn et al have also suggested that during plasma processing particles form and collect at the plasma/sheath boundary. At the end of plasma processing when the RF power is shut off, these particles fall onto the wafer. They further teach that this is a major source of particulate contamination of wafer surfaces in a plasma chamber. They suggested that the number of particles be reduced by turning off the RF power slowly, so that particles are released from the sheath region more slowly, so that they may be evacuated by the chamber exhaust system rather than drop onto the wafer. This solution however is uncontrollable at best.
It would be highly desirable to develop a reliable, inexpensive process to remove particles from wafer plasma processing chambers.